1. Field of the Invention
This invention relates to a one-way clutch for a torque converter used as a part for torque transmission, back stop, etc., in the driving apparatus of,-for example, an automobile, and more particularly to an improvement in a sprag.
2. Related Background Art
Generally, a one-way clutch comprises an inner race and an outer race radially spaced apart from each other and concentrically disposed for rotation relative to each, and a torque transmitting member disposed between the inner race and the outer race for transmitting torque, and the cam surface of a sprag meshes with the raceway surfaces of the inner and outer races to thereby transmit the torque.
In order to form an oil film, it is preferable that the cam surface of the sprag be as smooth as possible, but in order to give predetermined surface roughness to the cam surface, there is a sprag in which as described in Japanese Patent Application Laid-open No. H6-313445, minute unevenness is formed on a meshing surface to improve the forming rate of oil film on the cam surface.
The sprag, however, if the viscosity of lubricating oil becomes high under a very low temperature environment (e.g. −20° C. to −40° C. or lower), does not normally mesh but continuously causes slip when shift is made from idle rotation to meshing, and there may occur the inconvenience that the meshing of a sprag clutch does not function. When as in Japanese Patent Application Laid-open No. H6-313445, minute unevenness is formed on the surface, there is the possibility that lubricating oil of high viscosity collects in the unevenness and becomes oil film to thereby cause continuous slip.
To overcome these problems, it is conceivable to increase a ribbon spring force which applies a load in a meshing direction to the sprag, but an increase in the ribbon spring force may increase the drag during idle rotation, and may cause the aggravation of fuel consumption in a one-way clutch used in an automatic transmission.